This invention relates to a method of controlling the fuel supply to an internal combustion engine at acceleration, and more particularly to a method of this kind which is intended to improve the accelerability of the engine without spoiling the driveability at the beginning of acceleration of the engine.
A fuel supply control method for internal combustion engines is already known which is adapted to first determine a basic value of the valve opening period of a fuel injection device provided in the engine, i.e. the fuel injection quantity, as a function of engine rotational speed and intake pipe absolute pressure in synchronism with generation of pulses of a predetermined crank angle position signal, e.g. a top-dead-center (TDC) signal, and then correct the basic value thus determined by adding to and/or multiplying same by constants and/or coefficients being functions of parameters indicative of operating conditions of the engine such as engine rotational speed, intake pipe absolute pressure, engine coolant temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., to thereby control the air/fuel ratio of a mixture being supplied to the engine.
It is a general tendency with internal combustion engines that even when the fuel supply quantity is increased and accordingly the mixture is enriched in order to accelerate the engine, the rotational speed of the engine does not increase immediately upon the increase of the fuel supply quantity due to a time lag between the start of supply of such increased fuel quantity to the engine and actual increase of the engine output torque and accordingly actual increase of engine rotational speed. Such time lag is attributable not only to a time lag between the start of supply of the increased fuel quantity and explosive combustion of the mixture within the engine cylinders, but also to a detection lag of sensors for sensing the operating conditions of the engine, a time lag between opening action of the throttle valve and actual increase of the charging efficiency of the engine and according actual increase of the intake air quantity, etc. Particularly, in an internal combustion engine equipped with an electronically controlled fuel injection device, a large volume space is usually provided in the intake passage at a location downstream of the throttle valve for restraining fluctuations in the intake passage pressure to thereby minimize fluctuations in the intake air quantity. As compared with internal combustion engines equipped with carburetors, the above time lag between the supply of an accelerating increased fuel quantity to the engine and actual increase of the engine speed is conspicuous in such electronically controlled engine due to a longer period of time between opening action of the throttle valve and actual increase of the charging efficiency of the engine.
In order to compensate for a detection lag of the actual intake air quantity supplied to the engine at acceleration, it has conventionally been employed, for instance, to detect the opening speed of the throttle valve, set a value of a correction variable for increase of the fuel quantity on the basis of the detected opening speed, and supply a quantity of fuel increased by the set value of the correction variable. However, according to such accelerating fuel quantity control method, at the beginning of acceleration of the engine, that is, during a period of time after initial detection of acceleration of the engine and before several pulses of the aforementioned TDC signal are generated, the engine cannot have an increase in the output torque to a level required for the acceleration since there does not occur a sufficient increase in the charging efficiency before the lapse of the above period of time for the aforementioned reason. However, immediately when the charging efficiency and accordingly the actual intake air quantity has increased to such required level, the engine can undergo a sudden increase in the output torque. This sudden increase in the output torque causes rotational displacement of the engine body about its crankshaft. That is, while the engine body is generally mounted on a mount provided in a vehicle body, etc. via an elastic shock absorber formed e.g. of rubber, the torque increase causes an impact upon the engine mount to an extent beyond the limit of absorption of impact or shock by the shock absorber. This gives an unpleasant feeling of shock to the driver, etc.
Further, when the engine is accelerated from a decelerating state wherein the position of the engine body on the mount is usually biased toward the decelerating side with respect to its neutral position, the resulting amount of displacement of the engine body is large as compared with that obtained when the engine is accelerated from a cruising state, resulting in a large shock being given to the driver, etc. In addition, the presence of backlash of parts of the driving system of the vehicle such as the transmission gear forms a further factor for increasing the accelerating shock.